Many shielded electronic equipment enclosures and housings provide internal ventilation, temperature control and ambient pressure equilibration by air circulation through them. Air passage into and out of the enclosures is through electromagnetic-radiation-shielding devices and air filters located in the enclosure vents. It is expected of these shielding devices and filters that they prevent, or reduce to acceptable levels, passage of electromagnetic radiation or interference (EMI); and that they also prevent passage of particulate materials to specified size levels, while minimizing impediments to flow of air through the enclosure.
Many devices and assemblies that provide effective EMI shielding are known in the art and are commercially available. Likewise, air filtration devices and assemblies are well known and available commercially. There are also commercially available devices and assemblies in which the functions of EMI shielding and air filtration are combined in a single unit. For the most part, however, these devices and assemblies suffer deficiencies in size, weight, and cost that are desirable to be overcome.
For example, metal honeycomb assemblies are very effective in shielding against EMI, however, they are quite bulky, ranging in thickness from about 1/2" to several inches depending on the electromagnetic radiation frequency range over which they are providing shielding. Furthermore, the honeycomb assemblies are usually arrays of open tubes that provide no barrier properties to passage of dust, dirt, or other particles through them and, consequently, require that separate filtration units be included in the equipment if prevention of passage of particles into the enclosure is important. This adds to the bulk, cost, and complexity of the system.
Devices and assemblies which combine EMI shielding and filtration functions generally consist of layers of metal wire screen or mesh fixed in mounting frames. The mounting frames are usually metal extrusions having complex shapes to which the metal wire screen or mesh are joined by crimping or other mechanical means, or by joining methods such as soldering or welding. These, too, are quite bulky and, due to their complexity, difficult and expensive to manufacture and install.
In general then, for installations in which there is great flexibility in space and weight considerations, effective EMI-shielding filter assemblies can be constructed from methods and materials well known in the art.
On the other hand, for installations in small, portable electronic devices such as hand-held nuclear radiation detectors, hip-mounted air samplers, and other electronic sensors and instruments or, in mobile electronic systems such as radio/telephone communication systems, missile guidance and control systems, and the like, in which there are severe constraints on the size and weight of components, there is a need for compact, lightweight, effective EMI-shielding air-filtration devices and assemblies capable of operating in a broad range of environments.